1. Field of the Invention
The field of art to which this invention pertains is aromatic liquid extraction. More specifically, the invention relates to the use of an organic amine corrosion inhibitor in an aromatic sulfolane type organic compound liquid extraction process.
2. Prior Art
It is known in the art that a conventional process for the recovery of high purity aromatic hydrocarbons of, say, nitration grade from various feedstocks including catalytic reformates is liquid-liquid extraction utilizing a solvent such as diethylene glycol or sulfolane, each of which has high selectivity for the desired aromatic hydrocarbon components contained in the feedstock. Typically, in the practice of such prior art process a hydrocarbon feed mixture is contacted in an extraction zone with an aqueous solvent composition which selectively dissolves the aromatic component of the hydrocarbon feedstock thereby forming a raffinate phase comprising one or more non-aromatic hydrocarbons and an extract phase containing dissolved aromatic components. The extract phase is then separately distilled yielding an overhead distillate containing only a portion of the extracted aromatic component, a sidecut fraction comprising aromatic hydrocarbons and a bottoms fraction comprising lean solvent suitable for reuse in the extraction zone. Frequently to prevent losses of the solvent, the raffinate phase is washed with water in a washing zone in order to remove solvent from the raffinate phase.
Also, not infrequently, the extract phase is subjected to stripping or extractive distillation in order to remove a contaminating quantity of non-aromatic hydrocarbons from the extract phase. This stripping or extractive distillation operation is normally performed in order to make possible the recovery of nitration grade aromatic hydrocarbons such as benzene and toluene. Therefore, a typical prior art process for the recovery of aromatic hydrocarbons encompasses a solvent extraction step, a stripping or extractive distillation step, and a final distillation or recovery step for recovery of high purity aromatic hydrocarbons from the solvent phase. Another prior art step, more fully discussed hereinbelow, is a benzene distillation step which has as its function the recovery of benzene from the other aromatics.
The solvents which are applicable to the practice of the present invention and to the aromatics extraction process, generally, are known to be thermally unstable. The instability is not pronounced, however, and only becomes evident upon prolonged recycling of the solvent whereupon the accumulation of the decomposition products becomes evident. Generally, the rate of decomposition increases with increasing operating temperatures. Thus, it has been found that the rate of decomposition, for example, of sulfolane in an inert atmosphere is 0.002% per hour at 200.degree. C., 0.01% per hour at 220.degree. C., and 0.02% per hour at 230.degree. C. Similar thermal effects are observed with other satisfactory solvents and it is therefore desirable to keep temperature levels as low as possible. Accordingly, it is the practice, for example when using the sulfolane solvent system, to set a maximum process temperature of about 370.degree. F. while in the diethylene glycol solvent system it is the practice to set a maximum process temperature of about 380.degree. F. Consequently, the prior art defines such processing temperatures as being the point of thermal instability; although, it is known that there is some decomposition occurring below those temperature levels, and in some instances, temperatures above these temperature limits may be utilized for short periods of time. Similar points of thermal instability may be readily ascertained for other solvent systems.
It is known that the solvent decomposition results in the production of acidic organic deterioration products as well as polymerization products of a resinous character. It is further believed that the decomposition is accelerated by the presence of oxygen. The exact nature of the final decomposition products is not fully known, but where sulfolane is the solvent, the decomposition initially produces sulfur dioxide, sulfur trioxide, and olefins.
The presence of organic acids within the aqueous solvent and of sulfurous gases within an aqueous sulfolane solvent tends to cause accelerated corrosion of the steel materials used in the construction of the process unit, particularly when water is present which is usually the case. Therefore, it is the usual prior art practice to add organic amine compounds to the solvent composition as corrosion inhibitors. U.S. Pat. Nos. 3,385,783; 3,385,784; 3,466,345; and 3,642,614 all mention such use of organic amine compounds. Suitable organic amines for use in the solvent composition may be selected from the aliphatic, aromatic, naphthenic, and hetrocyclic amines, generally, as well as the alkanolamines containing one or more amine groups and/or hydroxy groups per molecule. The amine may also be a primary, secondary, or tertiary amine, but the preferred amine utilized as a corrosion inhibitor in the sulfolane solvent system is an alkanolamine, and more particularly, monoethanolamine. Because of the basic characteristics of these amine inhbitors, these materials react with the acidic solvent decomposition products to produce amine salts and amides at the temperature conditions utilized in the aromatic extraction process and thereby maintain solvent pH at a level far less conducive to corrosion.
The location in the aromatic extraction process at which the organic amine compounds are known in the art to be introduced is in the stripper receiver vessel. Addition at that location will provide inhibitor protection, of course, in the stripper overhead receiver, but also in the extractor, since hydrocarbons from the stripper are normally refluxed to the extractor. There is a problem, however, with the inhibitor addition as described above, because, due to loss in process product streams and consumption, very little inhibitor will be left in the process far downstream or upstream of the inhibitor addition point, particularly at the benzene column or benzene column overhead receiver, and, therefore, corrosion will occur at those downstream locations when solvent and solvent decomposition products are present. I have discovered a means of solving that problem.